Plant lipids have a variety of industrial and nutritional uses and are central to plant membrane function and climatic adaptation. The primary storage reserve of lipids in eukaryotic cells is in the form of triacylglycerols (TAGs).
TAG is the primary component of vegetable oil in plants and is used by the seed as a stored form of energy to be used during seed germination. The quality and content of plant oil can be altered by various methods, by impinging on the enzymes involved directly or indirectly in TAG biosynthesis.
Most free fatty acids become esterified to coenzyme A (CoA) to yield acyl-CoAs. These molecules are then substrates for glycerolipid synthesis in the endoplasmic reticulum of the cell where phosphatidic acid and diacylglycerol (DAG) are produced. Either of these metabolic intermediates may be directed to membrane phospholipids (e.g., phosphatidylglycerol, phosphatidylethanolamine, phosphatidylcholine) or DAG may be directed to form triacylglycerols (TAGS), the primary storage reserve of lipids in eukaryotic cells.
Sucrose is the major product of photosynthesis in higher plants and is transported from source to sink tissues through the phloem. It is also the major storage form of soluble carbon in sink tissues, and therefore also serves as a long-term energy source. Sucrose transporters (SUTs) play a major role in the photoassimilate accumulation in the sink tissues. The SUTs have been categorized into three major subfamilies: Type I (SUT1), Type II (SUT2) and Type III (SUT4). (Kuhn, Plant Diol (2003) 5: 215-232; Lim et al., Physiologia Plantarum (2006) 16: 572-584). Others have characterized sucrose transporters as high affinity/low capacity, low affinity/high capacity, and medium affinity/high capacity transporters.
Altering the expression level of sucrose transporters can be expected to have effects on the accumulation of photosynthetic assimilates in the sink tissues. Overexpression of a heterologous sucrose transporter has been shown to increase sugar content in sink tissues such as potato tubers, but does not lead to change in starch content or tuber morphology (Leggewie et al. Planta (2003) 217: 158-167). Tissue specific overexpression of heterologous sucrose transporters in storage parenchyma cells of pea cotyledons also increases sucrose influx into these cells, and increases the growth rates of pea cotyledons but does not lead to an increase in dry weight of fully developed cotyledons (Rosche et al., Plant Journal (2002), 30(2): 165-175).